1. Field of the Invention
The present invention relates to a pneumatic tire whose tread surface is formed with a block line comprising a plurality of blocks.
2. Description of the Related Art
Usually various tread patterns are formed on tread surfaces of pneumatic tires in accordance with required tire performance and using condition. Among the various tread patterns, a tread pattern called block type has a plurality of blocks divided by a plurality of grooves, and this tread pattern is known as one having excellent snow-covered road performance, drainage performance and traction performance.
FIG. 6 is a plan view showing one example of a conventional block type tread pattern. A tread surface 16 is formed with circumferential grooves 9 extending in the circumferential direction of the tire, inclined grooves 10 and inclined grooves 11 extending obliquely with respect to the circumferential direction of the tire, and block lines 19 in which a plurality of blocks 17 divided by the circumferential grooves 9 and the inclined grooves 10 and 11 are arranged. Each block 17 includes a tip end 18 which is tapered as viewed from above, and the tip end 18 is disposed between slits formed by the blocks 17 on opposite sides.
As shown in FIG. 7, the tip end 18 of the block 17 is formed with a taper cut portion 15 whose land portion height is gradually reduced toward the tip end side. The taper cut portion 15 is formed to solve a problem that the tip end 18 has small rigidity and it is prone to fall and the tip end 18 has relatively small ground pressure, the tip end 18 has small amount of wear as compared with other portions, so that the deviated wear is prone to be generated in the tread surface 16. By forming the taper cut portion 15, the wear amount difference is compensated to suppress the deviated wear.
According to the taper cut portion 15, however, it is not possible to sufficiently prevent the deviated wear from being generated. Especially when the angle of the tip end 18 is small, it is necessary to form a taper cut portion 15 having a great gradient, deviated wear resistance is not sufficient. Further, as shown in FIG. 6, if the tip end 18 is formed with the taper cut portion 15, a space provided between the slits becomes large, and there is a problem that deviated wear is easily generated along the circumferential direction of the tire in this region.
The following Japanese Patent Application Laid-open No. 2003-326920 discloses a pneumatic tire in which a tip end of the block is formed with a chamfered portion (corresponding to the taper cut portion). As the chamfered portion, there are described a first chamfered portion surrounded by a groove, and a second chamfered portion which is adjacent to a sidewall of the block. However, the first chamfered portion does not sufficiently prevent the deviated wear as described above. Further, in the second chamfered portion, since its inclined surface reaches the groove bottom, the rigidity of the tip end becomes excessively small, and the deviated wear resistance is not sufficient.